It appears that nobody here knows about Missingno., so let me break it down for you...
Nobody here has stated anything actually factual about Missingno., The Pokemon found on Cinnabar Island is known on 'M, NOT Missingno. It's a common error, but I am here to correct it. The method of encountering 'M was stated above, by Darmani Link. Once encountered, 'M will take the sixth item in your inventory and add 128 to it. (so if you had 3 potions in the sixth slot, you would now have 131.) 'M also screws up Hall of Fame data, and can potentially corrupt your file if caught...
Now, this brings up a good question... What is Missingno.?
Missingno. is another glitch Pokemon, however, it has five forms. all forms virtually do the same as 'M. (Take the sixth item in your inventory and add 128 to it, screws up Hall of Fame data, and can potentially corrupt your file...) Due to their similar actions, the two are commonly confused. Missingno.'s five forms are as followed...
-Normal (Looks exactly like 'M. Hint the confusion)
-Ghost (Looks like the spirits in Lavender Tower if you do not have the Silph Scope)
-Kabutops Fossil (Sprite of the Kabutops fossil found in the Pewter Museum)
-Aerodactyl fossil (Sprite of the Aerodactyl fossil found in the Pewter Museum)
-Yellow (looks like a glitched red and yellow box)
The only different between the forms are the ways you obtain them, they share the name "Missingno." and have the same effects to your file.
Go now, and and bask in your new-found knowledge...
You are aware that that pink bar only means that it's still contagious? Once a Pokemon has Pokerus, even when the pink bar disappears, the double EV effect still exists...
I believe the confirmed new number of Pokemon for Isshu is 156, bringing the total to 649, knowing that, how many of you can make 649 good, unique designs? I know I can't.
(Also, take off your Nostalgia Glasses, that may help...)